Structural characteristics of polysaccharides from olive fruit cell walls in relation to ripening and processing

Key words: Olive fruit; olive oil; pectic polysaccharides; xyloglucans; xylans;

enzyme preparations; phenolic compounds; processing; ripening

Technical enzyme preparations can be used as processing aids in the olive oil industry to obtain a higher yield and a better quality of the oil. These technical enzyme preparations degrade the plant cell wall, thus enhancing the permeability for oil. However, still very little is known about the specific role of the various constituent enzymes present in the preparations towards the polysaccharides in the plant cell wall. This study deals with this subject and describes the structural characteristics of the polysaccharides in the cell wall of olive fruit in relation to processing for oil extraction.

The major polysaccharides in the cell wall of olive fruit were found to be the pectic polysaccharides and the hemicellulosic polysaccharides xyloglucan and xylan. The pectic polysaccharides were highly methyl esterified and rich in arabinose. The xyloglucans had a backbone with three out of four glucose residues substituted with xylose residues and had a very specific substitution pattern, because galactose as well as arabinose residues could be linked to the xylose residues. The presence of arabinose residues linked to xylose residues is a common feature of xyloglucans produced by solanaceous plants, but has not been demonstrated for other dicotylodonous plants. Incubation of the cell wall material from olive fruit with pectin degrading enzymes in combination with endo-glucanases revealed that O -acetyl groups were not only linked to pectic material in olive fruit but also to xyloglucan. It was established that the arabinose residues of the xyloglucan could carry one or two O -acetyl groups. The cell wall material of olive fruit contained besides xyloglucan also considerable amounts of the hemicellulose xylan. These xylans appeared to be very low in substitution. Less than 10% of the xylose residues were mono substituted, mainly with 4- O -methyl-glucuronic acid residues.

The use of technical enzyme preparations during processing affected only a relatively small part of the polysaccharides in the cell wall. About 8-10% of the cell wall polysaccharides were extracted with cold and hot buffer and appeared to be modified during enzymatic processing. The changes of the buffer soluble pectic material were reflected by an increase in yield, a change in molecular weight distribution, a decrease in methyl esterification and a degradation of the (1®4)-linked galactan chains. No differences were observed in the composition of the arabinan chains during enzymatic processing. Analysis of the olive oil revealed that the use of enzyme preparations increased the concentration of phenolic compounds in the oil. Especially, the contents of secoiridoid derivatives such as the dialdehydic form of elenolic acid linked to 3,4-dihydroxyphenylethanol and an isomer of oleuropein aglycon increased significantly. These two phenolic compounds have high antioxidant activities.

The effect of enzyme treatment during the mechanical extraction of olive oil depends on the stage of maturity of the olive fruit. From this perspective the structural characteristics of the cell wall polysaccharides during ripening have also been investigated. During ripening the degree of methyl esterification and acetylation decreased and the solubility of the pectic polymers in buffer markedly increased. No distinct differences could be noticed with regard to the sugar composition and the profile of the molecular weight distribution of the pectic and hemicellulosic polymers during fruit development.